1. Technical Field
The invention relates to apparatus and processing entailing sub-micron imaging. An important aspect relates to the fabrication of small-dimensioned devices, e.g. integrated circuits using sub-micron design rules and to apparatus/tools used in such fabrication. While implications are broad, a major thrust concerns lithographic delineation--the use of phase masks to improve image quality. Whether based on presently used delineating energy, e.g. in the near ultraviolet spectrum, or on shorter wavelength, e.g. in the deep ultraviolet or x-ray spectra, improvement in lithographic delineation extends the range to permit further miniaturization. Fabrication of Very Large Scale Integrated circuits--electronic as well as optical and hybrid--built to sub-micron design rules is contemplated.
2. Description of the Prior Art
The saga of Large Scale Integration from inception to the present time is well-known. Evolution of the present 1-2 megabit chip, built to design rules at or slightly below 1 .mu.m, does not represent the ultimate product. Lithographic definition has played, and will continue to play, a significant role. Fabrication of state-of-the-art devices depends on use of near-ultraviolet radiation (e.g. of wavelength, .lambda.=3650 .ANG.--the mercury I line). Intensive effort directed toward next generation devices is expected to depend on radiation of still shorter wavelength (radiation within the "deep UV" spectrum, e.g. of wavelength, .lambda.=2480 .ANG.--the krypton fluoride excimer laser line). Forward-looking work directed toward still smaller design rules contemplates electromagnetic energy in the x-ray spectrum or, alternately, accelerated electron radiation of equivalent decreased wavelength.
A competing effort seeks to extend the capability of presently used UV delineating radiation. As described by M. D. Levenson et al, IEEE Trans. Electron Devices, vol. ED-29 (12), p. 1828 (1982) and as reviewed in a New York Times article dated Dec. 12, 1990, design rule-limiting edge resolution is lessened by use of "phase masks"--that is, masks designedly providing for relative phase shifting of radiation as transmitted through selected mask areas. Impact is two-fold: (1) as applied to usual device fabrication entailing opaque featured masks (e.g. chrome on glass) and (2) as applied to such fabrication entailing clear masks, dispensing with opaque mask features, in which use is made of dark-line imaging resulting from interference as between transparent mask regions of differing phase delay. In either event, use of phase masks permits extension to design rules generally thought beyond the capability of the particular wavelength used, with extension due to phase cancellation of diffraction-scattered radiation at feature edges. In both instances, provision is made for 180.degree. phase shift regions--either adjacent, or as an integral part of edge-defining mask areas.
Phase masking is considered promising in accordance with traditional business considerations. It permits fabrication of next-generation devices using present equipment and processing. Avoidance of cost of replacement equipment (in any event not yet commercially available), as well as of retraining of personnel assures continuing effort in this direction.
A widespread view serves as basis for expected extension of UV-based processing to design rules below 0.3-0.25 .mu.m by use of phase masking--likely to the 0.2 .mu.m and below range commonly thought beyond the effective capability of UV delineation. To the extent that this proves to be correct, device fabrication by use of x-ray (whether proximity or projection) as well as by use of accelerated electron radiation (whether by beam writing or masking) is likely to be deferred to the turn of the century.
Limiting lithographic resolution varies in accordance with the classical relationship: ##EQU1## in which: .lambda.=wavelength of delineating radiation in appropriate units, e.g. .mu.m NA is the numerical aperture of the optical system, Resolution is on the basis of desired feature-space contrast and K.sub.1 is a constant which depends upon details of the imaging system, and characteristics of the delineating process, e.g. of the development process--a value of 0.7-0.8 is representative of state-of-the-art fabrication (of 0.8-1.0 .mu.m design rule LSI).
180.degree. phase mask processing for given wavelength/etch contrast may be described in terms of reduction of K.sub.1 to the .apprxeq.0.5 level (permitting fabrication of devices to design rule of .apprxeq.0.4 .mu.m), and in some instances to the K.sub.1 .apprxeq.0.3 level to yield quarter micron features.